1. Field of the Invention
The present invention relates to a watt-hour meter for measuring electric power with the use of a multiplier which provides a digital output signal proportional to the product of input signal levels, and particularly to a multiplier which is suitable for integration.
2. Description of the Prior Art
FIG. 1 is a view showing a conventional multiplier which is applicable for a watt-hour meter for measuring electric power and provides a digital output signal proportional to the digital product of input signal levels. The multiplier shown in the figure is so constituted to provide a frequency proportional to the product of voltage values of two input signals.
In FIG. 1, an input signal is applied to an input terminal 1. A voltage of the input signal is modulated by a pulse width modulator 3 to provide a pulse signal. The pulse signal is applied to a switch 5 as well as being inverted in an inverter circuit 7 and applied to a switch 9. Namely, the switch 5 is opened and closed under the control of an output of the pulse width modulator 3, and the switch 9 is opened and closed under the control of an inverted output of the pulse width modulator 3.
An input signal to an input terminal 11 passes through the switch 5 and is smoothed in a smoothing circuit 16 which comprises a resistor 13 and a capacitor 15. The same input signal to the input terminal 11 is inverted and amplified in an inverter amplifier 23 which comprises an operational amplifier 17 and resistors 19 and 21. The inverted and amplified signal passes through the switch 9 and is smoothed in the smoothing circuit 16.
Namely, the input signal from the input terminal 11 and its inverted and amplified signal are selectively smoothed according to the pulse signal and its inverted signal which have been obtained by pulse-width-modulating and inverting the input signal to the input terminal 1. Thus, the input signals to the input terminals 1 and 11 are multiplied in time-sharing manner.
A multiplied and smoothed result from the smoothing circuit 16 is an analog signal which is converted in a voltage-to-frequency (V-F) converter circuit 25 into a frequency. Based on this frequency, a digital signal proportional to the product of voltage values of the input signals is obtainable.
In the application of this multiplier for a watt-hour meter, a digital signal proportional to the product of input voltage values is counted by a counter to display the counted result as electric energy.
Since the conventional multiplier applicable for a watt-hour meter analogously processes and multiplies input signals, it requires many parts such as capacitors and resistors to be assembled so that its size is unavoidably large to hinder its integration into an IC, particularly a custom IC.
To correctly measure electric energy, the capacitors of the multiplier must be accurate. The use of the switches 5 and 9 complicates the multiplier. If the switches 5 and 9 are semiconductor switches, their internal resistors may be affected by heat, to cause measurement errors.
Watt-hour meters presently marketed are well digitized by using microcomputers, etc. Therefore, it is desirable to digitize the multipliers.